1. Field of the Invention
The present invention relates to cables, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein at least one coating layer consists of a recyclable material which is halogen-free and has superior mechanical and electrical properties.
2. Description of the Related Art
There is currently a great need for highly environmentally friendly products, consisting of materials which are not harmful to the environment either during their production or when in use, and which are readily recyclable at the end of their working life. However, the option of using ecological materials is, in all cases, subject to the need to keep costs within acceptable limits, while still guaranteeing performances which are at least equivalent to those of conventional materials and which are, in any case, satisfactory under the most common conditions of use.
In the cables sector, in particular power transmission cables, the various coatings surrounding the conductor commonly consist of crosslinked polymer materials, in particular polyethylene or ethylene copolymers suitably crosslinked during extrusion, so as to give satisfactory mechanical performances even under heating in continuous use and under conditions of current overload, while at the same time maintaining a high level of flexibility. These materials are crosslinked and therefore cannot be recycled since they are devoid of thermoplastic properties, hence they can only be disposed of at the end of their working life by means of incineration. Moreover, in certain cases the outer protective sheath consists of polyvinyl chloride (PVC) which is difficult to separate by conventional methods (for example in water by density differences) from the crosslinked polyolefins containing inorganic fillers (for example from ethylene/propylene rubbers containing inorganic fillers), and, on the other hand, PVC cannot be incinerated together with crosslinked polyolefins since this produces highly toxic chlorinated products by combustion.
In U.S. Pat. No. 4,948,669 cable-coating compositions are described comprising from 29 to 50% by weight of low-density polyethylene, containing as comonomer an alpha-olefin having from 4 to 12 carbon atoms, in particular 1-octene, in an amount such as to obtain a density of between 0.90 and 0.92 g/cm.sup.3, in admixture with: (a) a propylene homopolymer; (b) a non-elastomeric copolymer of propylene with ethylene; or (c) heterogeneous copolymers of propylene with ethylene, obtained in reactor. As polyethylene it is particularly suggested using product Dowlex.RTM. 4000E from Dow Chemical, containing about 17% of 1-octene and having a melt index equal to 3.3 and a density of 0.912 g/cm.sup.3. These are products obtained using titanium-based Ziegler-Natta catalysts, having a relatively high density and thus little flexibility.
In patent application WO 96/23311 a low-voltage high-current cable is described, wherein the insulating coating, the inner sheath and the outer sheath are made of the same non-crosslinked polymer-based material which is black coloured by addition of carbon black. Using the same base material would allow recycling without the need to separate different materials. As polymer material for the outer sheath, it is suggested using, in place of PVC, ultra-low-density polyethylene (ULD-PE), for example products Engage.RTM. from DuPont-Dow Elastomers and Exxpol.RTM. from Exxon. Inorganic fillers such as aluminium or magnesium hydroxide are added to these materials in order to give them flame-retardant properties.
In U.S. Pat. No 5,246,783 cables are described, having as insulating and/or semiconductive coatings polymer materials based on copolymers of ethylene with at least one C.sub.3 -C.sub.20 alpha-olefin, with a density of from 0.86 to 0.96 g/cm.sup.3, known commercially under the tradename Exact.RTM. from Exxon, preparable using metallocene catalysts. These copolymers are used in crosslinked form, achieved by chemical means (for example with dicumyl peroxide) or by irradiation.